In any chromogenic photographic material it is desirable that the dyes formed should be bright in colour, absorbing light in the appropriate spectral region, with very little secondary absorption so that good colour reproducibility is obtained. While many of the desired characteristics are achieved by altering the molecular structure of the photographic dye-forming coupler, hereinafter coupler, much can be achieved by changing the environment of the coupler, and hence that of the azomethine dye which is formed during photographic processing. For example, the spectral properties of the dyes can be altered using hue-shifting solvents, such as sulfonamides (EP-A-0 309 159, EP-A-0 264 083), carbonamides (e.g. U.S. Pat. Nos. 4,935,321 and 5,258,278), aryl ureas (U.S. Pat. No. 4,808,502), ballasted and substituted phenols (U.S. Pat. No 4,562,146, DE 3,936,300), sulfones and sulfoxides (U.S. Pat. No 5,232,821) and phosphate esters (EP-A-0 515 128).
For economic and environmental reasons, there is an ongoing effort to reduce the quantities of materials used in photographic materials. Couplers derived from bicyclic or even tricyclic heterocyclic compounds are increasingly being utilised because they can form dyes with higher absorptivities than those that have been used for many years (such as pyrazolone magenta couplers and phenolic and naphtholic cyan couplers). Thus, on an equimolar basis, these heterocyclic couplers can provide much higher dye densities from the same amounts of silver, so that it is possible to generate the same dye densities from lower laydowns of silver and coupler.
However, heterocyclic couplers based on a pyrazole ring, such as pyrazolotriazole and pyrazolobenzimidazole couplers, that are increasingly being used in photographic elements, form magenta or cyan (or almost cyan) dyes which are highly aggregated, that is they form a self-associating assembly of monomeric molecules.
U.S. Pat. No. 5,294,528 lists a variety of agents which can be used to break the aggregation of an azomethine dye (more correctly an azamethine dye), in particular the dye from magenta pyrazolotriazole couplers. It is claimed in this patent that “azomethine dyes formed from pyrazoloazole couplers are liable to aggregate, and the higher the aggregation degree of the dyes is, the lower the light-fastness is, and that by breaking the aggregation the light-fastness of azomethine dyes can be enhanced”. Whilst it is true that some magenta pyrazoloazole couplers with bulky substituents provide dyes which are less aggregated and have better light stability than those with less bulky substituents, de-aggregation is not the main cause of improved light stability. A de-aggregating compound is understood to act by intervention within the dispersed oil phase containing the aggregated dye and is necessarily a component of the coupler dispersion. Compounds which de-aggregate in this way may perform quite different functions if included in separate dispersions, e.g. as UV absorbers. We have found that de-aggregating compounds do not necessarily provide extra light stability unless another appropriate light stabilizer is present. Thus improved light stability is not correlated with de-aggregation.
EP-A-0 886 179 suggests that liquid crystalline solvents can be used to improve colour reproducibility for a variety of heterocyclic coupler classes. EP-A-0 883 024 and U.S. Pat. No. 6,132,945 indicate that cyclic imide materials and phenyl carboxylic acid derivatives improve the colour reproduction of azomethine dyes produced by pyrrolotriazole couplers. U.S. Pat. No. 6,007,975 suggests that a phenolic coupler can be combined with various heterocyclic cyan couplers to improve colour reproduction.
2-hydroxyphenyltriazine materials are used as UV absorbers in plastics and in UV filter layers in photographic products (Swiss Patent Nos. 533853 and 557693); combined with magenta or yellow couplers to improve dye stability and Dmin yellowing (DE 4444258 A1 and U.S. Pat. No. 5,541,045), and used with phenolic and heterocyclic cyan couplers (other than pyrazolotriazoles) for dye stability improvements (DE 19538950 A1 and 19701869 A1). DE 19701719 teaches that distinct dispersions of stabilizer and coupler within the same layer can provide improved dark storage and light stability of the cyan image but additional dye hue improvements via de-aggregation of the dye are never mentioned.
U.S. Pat. No. 6,242,169 discloses a colour photographic material, containing a pyrazoloazole cyan coupler and a phenolic solvent, substituted in the para position with a nitrogen- or sulfur-bound group, the material having improved light stability. Although a phenoxy stabilizer may optionally be present, preferably in the same layer, there is no working example of its inclusion, no teaching that it would be in the same dispersion as the coupler, nor that hydroxy substitution would be favoured over, for example, alkoxy substitution.